Pain is the single most common symptom for which patients seek medical treatment and there is currently no objective method available for its measurement. Present methods of quantifying "pain" are little more than lexicons for its verbal description or biomechanical methods for measuring the restriction of a particular range of motion or activities of daily living associated with the pain. Some psychometric methods attempt to quantify the personality or cognitive distortions from which the pain patient suffers. In no case, however, do these methods reveal the covert and subjective sensory perception that is the pain experience in a way that can be quantified by an outside observer (for review, see Lipman J. J., "Pain Measurement," Pain Measurement: Contemporary, Issues in Chronic Pain Management, Ch. 9 (Kluwer Academic Publishers, Boston 1991)). The need for pain measurement methods was recently addressed by both the Social Security Administration and the United States Congress. A report ordered by Congress through the Secretary of Health and Human Services by a Commission on the Evaluation of Pain, recommended that some sort of objective measurement of pain be developed to assist in determining disability (see Fordice, Back Pain in the Workplace: Management of Disability in Nonspecific Conditions-Task Force on Pain in the Workplace, (I.A.S.P. Press, Seattle, 1995); Fields, Core Curriculum for Professional Education in Pain: Task Force on Professional Education, (I.A.S.P. Press, Seattle, 1995); American Pain Society, Principles Of Analgesic Use In the Treatment of Acute Pain and Chronic Cancer Pain-a Concise Guide, (American Pain Society, Wash. DC, 1990)).
The need for objective pain measurement goes beyond the economics of forensic disability assessment. Objective methods of pain measurement are required for accurate assessment of patient complaint and to assure appropriate treatment. For example, the need to appropriately medicate severe acute and chronic pain and also cancer pain requires an objective method of pain measurement. A corollary need is to avoid inappropriate treatment of pain--or claimed pain--where the possibility of malingering for secondary gain is a possibility. Such "secondary gains" are believed to account for an appreciable portion of chronic pain treatment demand, and forensically include the desire for disability payments, for insurance damage settlements or for other fiduciary incentives. Such secondary gains are not always conscious and may derive from psychological reasons related to the psychosocial set and setting of the patient and their disease. The inappropriate desire for opiate drugs probably accounts for a significant fraction of pain therapy prescription drug demand, yet absent any objective method of establishing the existence of "pain", the physician has no objective standards by which to prohibit such demand, and frequently feels ethically bound to take claims of pain at face value, or risk accusation of ineffective care and inhumane treatment.
Furthermore, an objective pain measurement device that is operable in the general practitioner's office would fulfill a pressing diagnostic need. It is from the general practitioner's office that referrals to neurologists are made. For example, patient complaints of subjective numbness are often not detectable on clinical examination because present diagnostic methods are not sensitive enough to detect the early stage sensory impairments of such neurological disorders as nerve root entrapment or peripheral neuropathy. As a result, patients with these types of neurological disorders cannot be diagnosed until the disorder progresses to a detectable level. The availability of a pain measurement device sensitive enough to detect the presence or absence of these and other abnormalities at an early stage would provide more effective medical intervention, or avoid unnecessary medical intervention. In order for such a device to be cost-effective for the general practitioner it should not require valuable dedicated space, and thus should be portable. Similarly, greater cost-effectiveness would be realized if the device were operable by a single person.
Basic psychophysical methods for the estimation of pain sensibility have a long history of questionable clinical relevance. Psychophysical methods seek to quantify pain intensity in an objective fashion despite the fact that pain is a complex and multi-faceted sensory mode, intrinsically containing dimensions of set, setting, ideation, memory, anxiety, and experiential import.
Subjective pain perception does not bear a simple relationship to stimulus intensity, but it nevertheless has some quantifiable dimensions and limits; a lower level of identity (the pain threshold) and an upper level of identity (the tolerance level). Below the pain threshold, stimuli of increasing intensity destined to broach this level are perceived as noxious yet non-painful (prepain). The pain threshold itself is highly labile and subject to psychological manipulation either of imposed suggestion (experimenter bias) or autosuggestion bias (the placebo response) or both. No studies have been able to demonstrate a relationship between pain threshold and the underlying pain state; in fact, pain threshold measurement procedures are unable to quantitatively demonstrate analgesic states engendered by clinically proven drugs as, for example, morphine (for review, see Chapman, et al. "On the Relationship of Human Laboratory and Clinical Pain Research," Pain Measurement and Assessment, pp. 251-257 (Raven Press, New York, 1983)). Furthermore, the method suffers from major disadvantages when transferred to the clinical situation where the test subject, who may suffer excruciating pain of endogenous pathological origin, is less able to attend to the minor sensory nuances of the pain threshold.
The pain sensitivity range constitutes a psychophysical region between the pain threshold level, where prepain becomes subjectively painful, and the pain tolerance level, which represents the greatest intensity of a noxious stimulus that a subject can tolerate (Hardy et al). In contrast to the pain threshold level, the pain tolerance level is subjectively distinct and unequivocal. Further, the pain tolerance level exhibits a linear change with stimulus intensity and yet it shares a sufficient commonality with the physiological processes of endogenous pathological pain perception that are positively influenced by changes in the endogenous pain state.
Pain tolerance levels are usually assessed by the use of a continuous, rather than a discrete, noxious stimulus, the cut-off of which is always the maximum limit of the subject's subjective pain tolerance. Pain tolerance has been measured by several means including the cold pressor test in which the hand or a limb is immersed in ice water until unendurable pain results, focal pressure, tourniquet ischemia and radiant heat (For review see Lipman J. J., "Pain Measurement," supra). Tolerance methods using these techniques, unlike threshold methods, also evoke some not inconsiderable anxiety and apprehension on the part of the subject, which may resemble the anxiety of the pain-suffering patient. However, studies have shown that tactile stimulation interferes with that aspect of cutaneous tolerance limit responsive to internal pain interference and thus methods that utilize a contact stimulus invalidate pain tolerance level results. While the cold pressor, focal pressure and tourniquet ischemia tests all involve tactile stimulation, radiant heat methods do not require direct contact with the subject.
The concept of a radiant heat pain stimulator for human use was initially developed by Hardy, Wolff and Goodell in 1952 (Pain Sensations and Reactions (Williams and Wilkins, Baltimore, 1952)). However, most radiant heat pain stimulators have been designed to measure the pain threshold level and thus are prone to the disadvantages inherent in measuring pain threshold. Recently, a concept prototype heat pain stimulator was developed that measures the pain tolerance level (see Lipman, et al., Pain 30, 59-67 (1987); Lipman and Blumenkopf, Pain 39, 249-256 (1989); and Lipman, et al., J. Neurosurg. 72, 883-888 (1990)). The concept prototype was a nonportable, electromechanical device that did not allow for automatic data acquisition. As such, the concept prototype required dedicated laboratory space and also required one person to operate the device and a second person to record data. Accordingly, there remains a need in the art for a non-contact, radiant heatbeam dolorimeter that provides a quantitative, objective measure of the pain tolerance level, is portable and allows for automatic data acquisition.